(1) Technical Field
The present invention relates to the fields of communications systems, and, in particular, satellite networks, signal power combining, and communication techniques for deep space operations. More specifically, but without limitation thereto, the present invention pertains to a communication system and method that allows a transmitter segment utilizing a unique multiplexing method to dynamically combine power from propagation channels in order to improve power levels of signals being transmitted without affecting the receiver segment (deep space spacecraft segment), the propagation segment, and without extensive overhaul of existing transmitting equipment (ground transmitting segment).
(2) Description of Related Art
Large antennas with aperture of 70 meter as part of NASA's Deep Space Network (DSN) allow the Voyager spacecrafts to send data from the far reaches of space. As these spacecraft move through space they are reaching distances of over 10 Astronomical Units (AU). Due to these enormous distances transmission times can take up to 14 hours. As these distances continue to increase the DSN is looking for ways to increase the signal strength received from these spacecraft. In 2006, The Space Communication Architecture Working Group (SCAWG) reported [1] that implementation of antenna arrays with 12 m reflectors as array elements would be an optimal choice for enhancing the DSN capability to meet evolving requirements and the increased demands for space communications services. Further more, the design challenges are in the power combining of the transmitting signals for up-link from hundreds of 12 m reflectors [2, 3, 4].
Conventional transmit beam forming network for active arrays with a large numbers of reflectors as array elements can produce super fine beams with high gain, featuring coherent “power combining” in far field ideally. The beam pointing directions for such a beam with super small beamwidth must be predicted and controlled accurately in order to “intercept” a spacecraft in deep space. However, the Ka band propagating effects through dynamic earth atmosphere do post “beam pointing challenges” to ground based large aperture arrays. These effects must be calibrated and compensated in order to form a super fine beam in far field for a fine pointing directions.
The key RF antenna performance characteristics of a typical 12 m reflector at Ka-band frequencies are listed in Table-1 at 26 GHz, 32 GHz, and 37 GHz of 3 possible NASA Ka band channels, and a typical Ka channel at 30 GHz. It is noticed that beamwidths of a 12-m dish at Ka band for the four identified frequencies are about 0.05° or less, and the far field distances are greater than 50 Km. Limited (electronic) scan angle ranges between ±0.3°. The scan range is defined as ±6 beamwidth in degrees for discussion purposes.
TABLE 1Single Dishdish size12m in diameterFrequency (GHz)26303237λ (cm)1.151.000.940.81πd/λ3267.263769.914021.244649.56aperture efficiency0.700.700.700.70Far field Distance (km)50586171Gain (dB)68.7369.9870.5471.80Beamwidth (milli degrees)55.0947.7544.7638.71Scannable range in ±deg.0.330.290.270.23(±6 beamwidth)
For an array with 20*20 reflectors, using 12-m reflectors as radiating elements, the array RF performance characteristics for the four identified frequencies are summarized in Table-2. It is assumed the array elements are on regular grids of square lattices. The assumed spacing are, respectively, 15 m for the array in case 1, and 25 m for the array in case 2. It is noticed that beam-widths of the 20*20 array occupying a 300 m*300 m area with 400 Ka dishes featuring 12-m apertures is about 0.002°, and the far field distance is around 36,000 Km; the altitude of GEO satellites. It is also noticed that beam-widths of the 20*20 array occupying a 500 m*500 m area with 400 Ka dishes featuring 12-m apertures is about 0.0011°, and the far field distance is around 100,000 Km; about 3 times the altitude of GEO satellites.
TABLE 220 * 20 arrayFrequency (GHz)263032371spacing15 mtotal array dimension300 * 300 m * mFar field Distance (km)31,200 36,000 38,400 44,400peak gain (dB)94.7696.0096.5697.82Beamwidth (milli degrees) 2.20 1.91 1.79 1.552spacing25mtotal array dimension500 * 500 m * mFar field Distance (km)86,667100,000106,667123,333peak gain (dB)94.7696.0096.5697.82Beamwidth (milli degrees) 1.32 1.15 1.07 0.93
Adaptive beam forming for receiving functions of a large array with fine beam width can be implemented to dynamic calibrate and equalize the Ka propagation effects through earth atmospheres. On the other hand, a significant challenge is the implementation of an array for transmitting (up-link). Conventional calibrations with pre-compensation using feedback information from a receiver, say, at a distance beyond 10 AUs become unpractical because of long round trip delays of >28 hours. Separated monitoring platforms will be used for the calibrations
It appears that key technical issues are related to high gain beams with very small beam-widths at Ka band. Specifically, the concerns are in the areas of feasibility of using conventional beam forming techniques to form a Ka transmit beam or coherent power combining toward targeted spacecraft (S/C), due to (1) dynamic differential propagation effects from earth atmosphere including troposphere, and (2) near field calibrations for far field performances.
For the foregoing reasons, there is a great need in power combining techniques for up-links of deep space communications; sending signals from ground based transmitters to spacecraft traveling in deep space.
Generally speaking the present invention is applicable for combining radiated power coherently from elements of any active antennas via WF muxing/demuxing techniques. The present invention for power combining on received data signal streams involves mechanisms and processing at a receiving end in a destination for coherent combining of radiations from various elements of an active array on a sender end which initiated the data signals stream. The signals concurrently radiated by different radiating elements of the active array non-coherently, will have various propagations effects. Embedded diagnostic/probing signals are used to dynamically equalize the differentials of amplitude, phase, and time delay due to propagations and imperfect or aging electronics for the signal amplifications and conditioning. In order to achieving the effects of power combining on a receiver, the present invention also involves restructures of signal stream to be transmitted on the sender end, referred to as data signals, into a multipath waveform structure and injections of pilot signals for diagnostic and probing into the same waveform structure; so that the data signals and the pilot signals are mutually orthogonal when traveling through the same multiple paths created by various radiating elements of the active array concurrently including various banks of parallel power amplifiers.
In order to achieving the effects of power combining, the embodiment of the present invention also involves restructures of signal stream to be transmitted on ground, referred to as data signals, into a multipath waveform structure and injections of pilot signals for diagnostic and probing into the same waveform structure; so that the data signals and the pilot signals are mutually orthogonal when traveling through the same multiple paths concurrently including various banks of parallel power amplifiers.
The following references are presented for further background information:    [1] D. Chang, W. Mayfield, J. Novak III, and F. Taormina, “Phased Array Terminal for Equatorial Satellite Constellations,” U.S. Pat. No. 7,339,520, Mar. 4, 2008; and    [2] D. Chang, W. Lim, and M. Chang, “Multiple Dynamic Connectivity for Satellite Communications Systems,” U.S. Pat. No. 7,068,616, Jun. 27, 2006.    [3] The Final Report of “NASA Space Communication and Navigation Architecture Recommendations for 2005-2030” by Space Communication Architecture Working Group (SCAWG), 15 May 2006    [4] Morabito, D. D. (2007), Detection of tropospheric propagation effects from deep space links of the Cassini spacecraft, Radio Sci., 42, RS6007, doi:10.1029/2007RS003642.    [5] F. Mantovani and A. Kus; “The Role of VLBI in Astrophysics, Astrometry and Geodesy”, Academic Publishers: P. 383-401. “Tropospheric and ionospheric Phase calibration” by J.-F. Lestrade    [6] D. Bilitza, International Reference Ionospheric Model, 2000 e2001, http://modelweb.gsfc.nasa.gov/models/iri.html.